Ion implanters are widely used in semiconductor manufacturing to selectively alter conductivity of materials. In a typical ion implanter, ions generated from an ion source are directed through a series of beam-line components which include one or more analyzing magnets and a plurality of electrodes. The analyzing magnets select desired ion species, filter out contaminant species and ions having incorrect energies, also adjusting ion beam quality at a target wafer. Suitably shaped electrodes can be used to modify the energy and the shape of the ion beam.
FIG. 1 shows a known ion implanter 100 which comprises an ion source 102, extraction electrodes 104, a 90° magnet analyzer 106, a first deceleration (D1) stage 108, a 70° magnet analyzer 110, and a second deceleration (D2) stage 112. The D1 and D2 deceleration stages (also known as “deceleration lenses”) are each comprised of multiple electrodes with a defined aperture to allow an ion beam to pass therethrough. By applying different combinations of voltage potentials to the multiple electrodes, the D1 and D2 deceleration lenses can manipulate ion energies and cause the ion beam to hit a target wafer at a desired energy.
The above-mentioned D1 or D2 deceleration lenses are typically electrostatic triode (or tetrode) deceleration lenses. FIG. 2 shows a perspective view of a conventional electrostatic triode deceleration lens 200. The electrostatic triode deceleration lens 200 comprises three sets of electrodes: entrance electrodes 202 (also referred to as “terminal electrodes”), suppression electrodes 204 (or “focusing electrodes”), and exit electrodes 206 (also referred to as “ground electrodes” though not necessarily connected to earth ground). A conventional electrostatic tetrode deceleration lens is similar to the electrostatic triode deceleration lens 200, except that a tetrode lens has an additional set of suppression electrodes (or focusing electrodes) between the suppression electrodes 204 and the exit electrodes 206.
In the electrostatic triode deceleration lens 200, each set of electrodes may have a space to allow an ion beam 20 to pass therethrough (e.g., in the +z direction along the beam direction). As shown in FIG. 2, each set of electrodes may include two conductive pieces electrically coupled to each other to share a same voltage potential. Alternatively, each set of electrodes may be a one-piece structure with an aperture for the ion beam 20 to pass therethrough. As such, each set of electrodes are effectively a single electrode having a single voltage potential. For simplicity, each set of electrodes are referred to in singular. That is, the entrance electrodes 202 are referred to as an “entrance electrode 202,” the suppression electrodes 204 are referred to as a “suppression electrode 204,” and the exit electrodes 206 are referred to as an “exit electrode 206.”
In operation, the entrance electrode 202, the suppression electrode 204, and the exit electrode 206 are independently biased such that the energy of the ion beam 20 is manipulated in the following fashion. The ion beam 20 may enter the electrostatic triode deceleration lens 200 through the entrance electrode 202 and may have an initial energy of, for example, 10-20 keV. Ions in the ion beam 20 may be accelerated between the entrance electrode 202 and the suppression electrode 204. Upon reaching the suppression electrode 204, the ion beam 20 may have an energy of, for example, approximately 30 keV or higher. Between the suppression electrode 204 and the exit electrode 206, the ions in the ion beam 20 may be decelerated, typically to an energy that is closer to the one used for ion implantation of a target wafer. Therefore, the ion beam 20 may have an energy of, for example, approximately 3-5 keV or lower when it exits the electrostatic triode deceleration lens 200.
The significant changes in ion energies that take place in the electrostatic triode deceleration lens 200 can have a substantial impact on a shape of the ion beam 20. FIG. 3 shows a top view of the electrostatic triode deceleration lens 200. As is well known, space charge effects are more significant in low-energy ion beams than in high-energy ion beams. Therefore, as the ion beam 20 is accelerated between the entrance electrode 202 and the suppression electrode 204, little change is observed in the shape of the ion beam 20. However, when the ion energy is drastically reduced between the suppression electrode 204 and the exit electrode 206, the ion beam 20 tends to expand in both X and Y dimensions at its edges. As a result, a considerable number of ions may be lost before they reach the target wafer, and the effective dose of the ion beam 20 is reduced.
There have been attempts to reduce the above-described space charge effect in an electrostatic triode lens. In one approach, for example, Pierce geometry, well known to those skilled in the art, is introduced to each electrode in the electrostatic triode deceleration lens. That is, each electrode is bent at its tip to a defined angle such that electric fields inside the electrostatic triode lens are such that they generate focusing forces counteracting the space charge spreading effects at the edge of an ion beam. However, this approach can only achieve a limited success in controlling ion beam shapes. Despite a changed shape, each electrode still remains one conductive piece biased with a single voltage potential. As a result, generation of the focusing forces at the edge of the ion beam is constrained by the overall voltage potential applied to the electrode. In addition, one particular shape of an electrode may be useful for adjustment of only one particular beam shape or the purveyance of the ion beam.
In view of the foregoing, it would be desirable to provide a technique for providing an electrostatic lens which overcomes the above-described inadequacies and shortcomings.